High-pressure gaseous discharge lamp



Patented Feb. 23, 1954 George-A. Freeman, East Orange, and Edward A.

Jenkins, Eairlawn, N. J., assignors house. Electric. Corporation, East.

ta Westing- Pittsburgh,

Pa., a corporation. of Pennsylvania Application August I2, 1949, Serial No. 109,902 .1; 1(Cl. lid-50:54)

1.: The present invention relatesto-- high pressure gaseous discharge lamps andmore particularly to aseal therefor between the vitreousen-velope and a high current carrying leading-in conductor for the electrodes.

It is well recognized inthe art that high pressure gaseous discharge lamps reach a relatively hightemperature during operation which, together with a high pressure of many atmospheres, present an exceedingly difiicult problem from the standpoint of maintaining an hermetic seal. In-

an attempt tosolve this problem seals have been employed which are capable of carrying several hundred amperes of current by utilizing a-number of parallel metallic strips of molybdenum foil. Seals. of this type are di-fiicult to make and exceedingly expensive because of the painstaking skill required for their fabrication.

Another attempted solution has been the use of. graded'seals, but. this requires that one end of. the seal be formedof a glass having a coefli cient. of expansion approximately that of the metallicv leading-in conductor which is usually formed of. tungsten. The objection inherent in such prior: art graded sealshas been that of the glass approximating the expansion of the-tungstenis entirely toosoft and hence will not withstand thetemperature to which the lamp is subjected during operation. To overcome this lat ter-difliculty', special high melting point glass has been developed, but which retains a coefiicient i expansion. approximating that of tungsten. Again,.however, an objection hasv been found to be-inherent in this type ofv graded seal, because the size of the graded. seal makes a large cavity in. thelamp which is. di-fiicult to keep at a sufiiciently high temperature to prevent condensation of mercury vapor, most commonly employed as the. discharge supporting medium, during operation of the lamp. Moreover, during flame working. seals of this type crack too readily during. fabrication of the lamp, thus increasing salvage costs.

It is accordingly an object of the present invention to provide a seal for a high pressure discharge lampcapable of carrying high current during operation which will maintain an hermetic seal without destruction at the high operat'i-ngtemperature of such lamp and constructed in such a way as to avoid a large cavity in which mercury might condense.

Another object of the present invention is the provision of a graded seal for high pressure discharge lamps to bring in leads of large crosssection capableof carrying high current and of withstandingthe high temperature to which the lamp is subjected duringoperation, without destruction of the seal and constructed in suchaway as to avoid a largecavity inwhich mercury might condense.

A further object of the present-invention is the provision of a short are high pressure discharge lamp provided with agraded seal for the leading-in conductors which will withstand-the high pressure and high temperature to which thelamp is subjected duringoperation without de struction of such seal and constructed in such a way as-to avoid a large cavity in which mercury might condense.

Still further objects of the present inventionwill become obviousto those skilled in the art by reference to the accompanying drawing wherein:

Fig. 1 is an elevational view of a high pressure discharge lampprovided with hermetic seals in accordance with the present invention;

Fig. 2 is a cross-sectional view of the hermetic seal assembly of the present invention;

Fig. 3 is a cross sectional view taken on the line III-III of Fig. 2;

Fig. 4' is a cross-sectional view similar to Fig. 2, but showing an intermediate fabrication ste in the manufacture of the seal assembly; and

Fig. 5 isa cross-sectional View similar to Fig. 2, but showing amodification which the seal of the present invention may take.

Referring now to the drawing in detail, the lamp shown inFig. 1 comprises a vitreous envelope 5 capable of withstanding high operating temperatures, such for example, as quartz. A pair of oppositely disposed refractory meta-l electrodes 6 and I; such as tungsten or the like, are secured to the leading-in and supporting conductors 8- and 9, by a refractory metal fastening collar l8. Such collar is may besecuredto the leading-in conductor and the electrode in any suitable manner, as by a threaded connection, set screws, or by welding. As is customary in the art following exhaust of the lamp at the tip I2; it is filled with a small quantity of an inert gas, such as neon; argon, or a mixture thereof, at a pressure of about 50 mm. to facilitate the initiation of a discharge between the oppositely disposed electrodes 6 and I.

In addition a. small quantity of a vaporizable material, such as mercury i3, is disposedin the envelope 5. which supports the arc-discharge once the lamp reaches stabilization temperature ranging from.50'0 C. tov 706 C., at. whichtime the mercury will be. completely vaporized. and, at.

" a pressure ranging between Sand 10 atmospheres.

Since the lamp as shown in Fig. 1 is a high intensity light source, with a power input of approximately 7.5 kw., the leading-in and supporting conductors must necessarily carry a comparatively high current of many amperes and accordingly are of relatively large cross-sectional area. The leading-in conductors for the lamp as shown are thus approximately one-quarter inch in diameter and may be of molybdenum, as shown in the preferred form of Figs. 1 to 4, or of tungsten, as shown in Fig. 5.

For the purpose of forming an hermetic seal between the leading-in conductor and the quartz envelope, the seal is first formed as a preliminary assembly by providing the leading-in conductor with a glass bead i l having a coefiicient of expansion approximating that of the molybdenum rod l5, or the tungsten rod It of Fig. 5. The rod is then provided with a thin metallic layer of a high refractory metal, such as tungsten, platinum, thorium or molybdenum, which may be in the form of a thin split sleeve or, as shown, it may be formed of a helical wrapping of metallic foil strip ll, such as molybdenum of approximately .0005 thickness, which in either case extends for an appreciable distance along the length of the rod IE or IE with such wrapped foil also extending a slight distance over the surface of the glass bead Hi. A vitreous sleeve l8 having an enlarged end I9, and an internal bore slightly greater than the diameter of the wrapped rod, is then slipped over the latter, as shown in Fig. 4. It will be noted that such sleeve i8 is formed of a plurality of glass sections having a graded coefficient of expansion as is well known in the art, with the upper section 20 as shown being of the same composition as the relatively soft glass bead 54, while the lowermost section 22 is formed of a harder glass which has a coefiicient of expansion approximating that of the quartz end portion IS.

The glass sleeve ill, or specifically the uppermost glass section 20 ofthe graded seal, is first sealed by fiame working to the glass bead [4 which is of a softer vitreous material than the envelope 5 and, since a few turns of the metallic foil l'l slightly overlaps the bead It, the end of the metallic foil is thus completely embedded in the fused mass of solid relatively soft glass, as can be observed from the completed seal assembly as shown in Figs. 2 and 5. The glass sleeve is then connected at its enlarged end 19 to a vacuum pump and after evacuation is tippedofi from the pump, as shown at the tip 24 of the enlarged end H9 in Fig. 5. Following such exhaust of the assembly, the sleeve 18 is heated by a sealing flame until the wall thereof, together with its integral graded seal, is shrunk down to the metallic foil strip 11, as can be again observed from the finished seal assembly of Figs. 2 and 5. Such shrinking causes the sleeve l8 and its graded seal to wet and adhere to the metallic foil strip i! which being of very thin cross-section, as previously mentioned, will yield slightly thus avoiding production of destructive strains and forming a graded hard glass buffering seal.

Moreover, the leading-in conducting rods 8 and 9 can move slightly under the foil wrapping I! to accommodate differences in thermal expansion between the rod and the quartz and seal glasses, without imposing a destructive strain upon the latter. Since the foil ll extends to slightly overlap the glass bead 14 to which the uppermost glass section '20 is fused, as above noted, an hermetic seal is thus formed at such fused juncture which is devoid of any sharp crevice that would otherwise be conducive of a slight strain causing cracking of the seal at such point.

In addition, the formation of a cavity in the seal, where at low temperature mercury vapor would otherwise condense, is efiectively eliminated due to the close adherence of the shrunken glass sleeve and graded seal to the metallic foil strip H and the latter to the leading-in conductors 8 and 9, for substantially the entire length of the shrunken sleeve and graded seal.

Following such shrinking of the glass sleeve and graded seal, the flanged end 19 of the seal assembly is cut at its enlarged end leaving a peripheral edge 25 as shown in Fig. 2. The electrodes 6 and 7, respectively, are then secured by means of the collar H] to the end of the leading-in conductors or rods 8 and 9, thus completing the preliminary seal assembly. The latter is then ready for fusing at the peripheral edge 25" of the flanged portion 19 to an annular vitreous collar or extension on the bulbous envelope 5, thus forming a juncture 23 which is hardly observable in the finished lamp of Fig. 1. The lamp is then completed in the customary manner by again evacuating, filling with a small quantity of a monatomic starting gas, such as argon, neon or a mixture thereof, and a small quantity of a vaporizable material, such as mercury, in an amount which becomes completely vaporized during operation and the lamp finally sealed oil.

The modification as shown in Fig. 5 differs from that shown in the remaining figures, not only in that the leading in conducting rod is shown as formed of tungsten with a glass bead HS and uppermost section ll of a glass having a coefficient of expansion approximating that of melting point sealing glasses are much more dif-- ficult to work and since more skill is required, it is more desirable to employ the softer glass graded seals, as shown in the preferred embodiment of Figs. 1 to 4, inclusive. Of course, in instances where the leading-in rod is of molyb-' denum, the seal may also be of shorter length when desired by employing the higher melting point sealing glasses, so long as the thermal ex-' pansion of beading glass 14 matches that of the" metallic rod, whether it be tungsten, molybdenum it will be understood that still further modifica-.

tions thereof may be made without departure from the spirit and scope of the appended claim.

We claim:

An hermetic electrically conducting seal for an electrical discharge lamp comprising an elongated rod like molybdenum leading-in and supporting conductor carrying an electrode at one end thereof, a thin spiral wrapping of molybde- 5 num ribbon surrounding said leading-in and Supporting conductor and firmly engaging the surface thereof throughout substantially its en tire length within the lamp, and a vitreous sleeve including a graded portion formed of a plurality of vitreous sections having a graded coflioient of expansion and fused at one end of said s1eeve to said leading-in conductor at a point remote from the electrode carrying end and embedding one end of said metallic wrapping in the fused mass, and said graded portion of said vitreous sleeve extending a substantial distance longitudinally of said leading-in conductor and being shrunken down into wetted engagement with said wrapping substantially throughout its length, to form a graded buffering seal and to effectively eliminate the formation of a cavity in the seal.

GEORGE A. FREEMAN. EDWARD A. JENKINS.

References Cited in the file of this patent UNITED STATES PATENTS Number 

